As a compound adjective, 6-page is correct.
a 6-page document
I cannot see a time when you would need "6-pages".
This document has 6 pages. (< Note, no hyphen.)
A hyphen can be used to link words together to show they are part of the same adjective. That's called a compound adjective.
6-page is right. We usually do not say "I read a 6-pages essay on Indian English". We say "I read a 6-page essay on Indian English". But you must say "That essay has 6 pages."
Why NOT say that "that essay has 6 pages"-is it because any number under 10 ( 1, 2, 3, 4, 5, 6, 7, 8, 9) you write in letters, whereas anything above you can simply write the digits?
Using AP style you would spell out six, too.